Vehicle occupant air bag restraint systems include an accelerometer or array of accelerometers for supplying signals to an electronic processing unit for deriving an output signal in response to a sensed vehicle deceleration associated with a crash. The electronic processing unit derives a crash indicating output signal that is supplied to a pulsed gas source. The pulsed gas source fills an air bag that is inflated in a vehicle occupant compartment against the body of an occupant, to hold the occupant in place during the deceleration forces associated with a vehicle crash The bag must be filled with approximately 100 liters of gas to a pressure of three to four atmospheres from the pulsed gas source in approximately 40 milliseconds. It is essential for the gas from the pulsed gas source to decrease quickly to approximately ambient level to enable the occupants of the vehicle to escape from the vehicle, if necessary. It is also necessary for the gas of the pulsed gas source to be non-toxic and non-combustible because (1) the gas in the bag ultimately vents into the vehicle occupant compartment on deflation and (2) the possibility of air bag failure during a crash or during inadvertent inflation in a non-crash situation. It is also necessary for a gas generating mass of the pulsed gas source to remain stable over long time durations and under fairly extreme operating temperature conditions of between -45.degree. C. and 70.degree. C.
The first vehicle occupant air bag restraint systems used high pressure stored gas to inflate the air bag. While these systems adequately inflated the air bag in response to a crash condition being sensed, they had numerous disadvantages relating to weight, size, cost and reliability. An exemplary prior art vehicle occupant air bag restraint system of this type is disclosed in U.S. Pat. No. 3,837,671.
A second vehicle occupant air bag restraint system which is currently being extensively used is of the type disclosed in U.S. Pat. No. 4,929,290 wherein the high pressure gas pulse is derived from a solid propellant, usually sodium azide (NaN.sub.3). The sodium azide is burned in response to burning of black powder which is ignited by a sufficient current being supplied to a fusible metal element embedded therein to cause the element to fuse and explode the propellant to generate the high pressure gas pulse. To prevent the partially-combusted materials from injuring vehicle occupants who are in the air bag path, a gas filter is located between the propellant and folded air bag, to pass gas from the pulse gas source, while blocking the flow of particulates.
Despite the wide use of this technique, the sodium azide propellant has numerous disadvantages. Sodium azide manufacture is hazardous because of a substantial risk of accidental fire and explosion, at least until the propellant is pelletized In addition, when sodium azide is ignited it can produce harmful by-products and is likely to produce partially-combusted materials that can burn through fabric of an air bag. Further deficiencies in the use of sodium azide as a propellant for deriving the high pressure gas pulse of a vehicle occupant air bag restraint system relate to size, weight, and cost. In addition, sodium azide is a carcinogen which can have possible detrimental effects on vehicle occupants and personnel who assemble the air bags.
Another high pressure pulsed gas source that has been proposed in vehicle occupant air bag restraint systems is disclosed in U.S. Pat. No. 3,966,266. In this system, there is a combination of a stored gas source and a combustible propellant. The propellant is ignited and the gas generated thereby is supplemented by the stored high pressure gas. This compromise system suffers, to a certain extent, from the deficiencies of the two previously mentioned systems.
It is, accordingly, an object of the present invention to provide a new and improved high pressure pulsed gas source and method of a type particularly adapted for use with vehicle occupant air bag restraint systems.
Another object of the invention is to provide a new and improved pulsed gas source and method having a programmed, controlled and predictable pressure versus time variation. A programmed pressure vs. time variation for an air bag is particularly advantageous because it enables the same basic structure to be used with different automotive vehicle models and for different occupant locations in the same vehicle. The passenger air bag is generally larger than the driver air bag because the passenger is much more likely to be in many different positions than the driver.
It is important in vehicle occupant air bag restraint systems for the gas supplied to a constant volume that is approximately twice the volume of a filled air bag to have a controlled pressure versus time variation to achieve proper air bag filling. It is preferable for the gas pressure versus time variation to have an increasing slope over a substantial portion of the gas pulse duration and for the slope not to decrease until shortly before the pulse terminates, i.e. as deflation of the air bag begins. Hence, during the early part of the inflation operation it is desirable to have a slow pressure increase (i.e. low slope), so that the occupant does not receive an initial possibly injurious high impact from the bag. This is particularly important for small occupants, e.g. children, or passenger seat occupants who are likely to be in many different positions at the time of a crash. As time progresses it is desirable for the slope of the pressure vs. time variation to increase to enable the occupant to be firmly secured in place to minimize injury.
It is also desirable in vehicle occupant air bag restraint systems for the gas pressure to be controlled as a function of temperature. If a crash occurs at low temperature, it is preferable to supply a greater number of gas molecules to the air bag over the gas pulse interval required to fill the air bag, as a result of the basic gas laws. Conversely, if a crash occurs at high temperature the air bag should be supplied with fewer gas molecules over the 40 millisecond interval. In the prior art, this desired result has not been possible.
In the prior art, at low temperatures, the bag generally is not fully inflated during the 40 millisecond interval because the number of propellant gas molecules has a tendency to remain the same at low temperature as at room temperature. In the opposite manner, at high temperature the bag becomes full within less than 40 milliseconds and has a tendency to exert excessive force against the occupant, with possible injury to the occupant.
It is, accordingly, still another object of the present invention to provide a new and improved pulsed gas source particularly adapted for use in vehicle occupant air bag restraint systems, wherein the pressure derived from the source supplied to a constant gas volume that is approximately twice the volume of a filled air bag has an increasing slope over a majority of the gas pulse duration.
An additional object of the invention is to provide a new and improved pulsed gas source and method particularly adapted for vehicle occupant restraint systems wherein the number of gas molecules supplied by the gas source changes inversely as a function of temperature.
It has also been proposed to activate an air bag by supplying a plasma to a relatively inert slurry of aluminum and water. The plasma is derived by supplying sufficient energy to a fuse wire to cause the wire to rupture and establish a plasma arc. However, because of the large amount of energy necessary to convert an aluminum water slurry to a high pressure vapor, such an air bag actuator is not practical for automotive vehicles having the typical 12-volt DC power supply including a battery and an alternator.
It is, accordingly, still another object of the present invention to provide a new and improved pulse source, particularly adapted to activate an automotive vehicle air bag in response to a plasma being supplied to a gas generant.